The long-term goal is to elucidate the processes leading to trisomy, the most frequent chromosomal anomaly among human conceptions and a major cause of pregnancy loss and severe mental retardation. Advancing maternal age is the only established risk factor. This project (a) tests whether the association reflects accelerated aging in the ovary and (b) examines connections with a genetic characteristic associated with premature ovarian aging. We hypothesize that trisomy arises as a function of the size of the oocyte pool (ovarian age), with trisomy risk increased among women with smaller pools at any given chronologic age. The number of oocytes is highest before birth and decreases as women age. At any given chronologic age, the number varies from woman to woman. Small pools may result from diminished production of oocytes during fetal development or accelerated oocyte atresia (loss). Little is known about the causes of either process. One possible cause of a smaller oocyte pool is the size of the CGG repeat on the FMR1 gene, which has been associated with premature ovarian failure. We hypothesize that FMR1 repeat size is associated with trisomy. This application draws on already collected data. For tests of the oocyte pool hypothesis, the primary data derive from 100 women with trisomic pregnancy losses (cases) and 279 controls with live births, as well as two subsidiary comparison groups with pregnancy losses. Hormonal indicators of the size of the oocyte pool include anti- Mullerian hormone, follicle stimulating hormone and inhibin B. For tests of associations with FMR1 repeat size, the sample comprises 206 trisomy cases and 548 controls, as well as two subsidiary comparison groups. The specific aims are: 1. To test whether hormone levels indicate more advanced ovarian age among trisomy cases than among live birth controls of the same chronologic age and to assess the specificity of associations. 2. To test whether maternal FMR1 repeat size is longer in trisomy cases than in controls. 3. To elucidate mechanisms by examining the relation of FMR1 repeat size to hormonal indicators. Confirmation of the hypotheses has implications both for research on mechanisms of trisomy formation and potential application for counseling women about pregnancy. This application seeks to elucidate the processes leading to trisomy, the most frequent chromosomal anomaly among human conceptions and a major cause of pregnancy loss and severe mental retardation. It tests the hypothesis that the strong association with maternal age reflects accelerated aging in the ovary. This project has important clinical implications for counseling women who wish to delay childbearing to older ages.